The present invention relates to electrodes useful for measuring properties of fluids, particularly body fluids. The present invention is particularly concerned with a microelectrode capable of simultaneously measuring the pH, P.sub.CO2, and P.sub.O2 of body fluids.
Various types of electrodes have been introduced in the past to measure pH, P.sub.CO2 and P.sub.O2. The first pH-sensitive electrode used in biological applications was a miniature platinum-hydrogen electrode. This electrode suffered from several disadvantages that included very slow response time, and a voltage response which was sensitive to partial pressure of hydrogen and several oxidizing agents.
An antimony-based electrode has been used to measure the pH in chicken eggs. This electrode works by the occurrence of an oxidation-reduction reaction on a thin layer of antimonous oxide on the electrode surface. Drawbacks associated with this electrode include a pH sensitive reaction, which is not completely reversible, a voltage response which is temperature dependent and a requirement that calibration solutions be used which closely resemble the ionic composition of the systems studied. In view of the difficulties associated with metal electrodes, in the past 20 years pH-sensitive glass electrodes have become very popular.
pH-sensitive glass has been used to fabricate pH microelectrodes. Numerous improvements have been made to these glass pH microelectrodes with one of the most important improvements being introduction of the recess tip pH microelectrode. These microelectrodes work well for both intra and extra cellular pH measurements, unfortunately these microelectrodes are difficult to fabricate.
P.sub.CO2 microelectrodes are available, which consist of an outer glass pH-insensitive shell into which a glass membrane pH electrode is placed in a weak bicarbonate solution behind a carbon dioxide permeable silicone rubber in the electrode tip. The P.sub.CO2 is thus measured, as a pH change of the filling bicarbonate solution caused by the migration of CO.sub.2 into the bicarbonate solution through the CO.sub.2 permeable silicone rubber. This electrode works well in tissue/body fluids but is also very difficult to fabricate. An antimony microelectrode has also been used to measure the P.sub.CO2 of body fluids, but this electrode also suffers from all the inherent difficulties of the antimony pH electrode discussed above, in addition to the difficulty involved in fabricating such electrode.
Because the pH, P.sub.CO2, and P.sub.O2 are of vital importance to organ functions, a microelectrode device which is capable of simultaneously measuring all three parameters would have a wide application in research medicine. It could be used to monitor the tissue properties while studying an organ's susceptibility to injury induced by decreased blood flow. Such determinations would be useful in studies such as those related to acute renal failure, a major cause of kidney shutdown in hospitalized patients. Also, electrodes capable of simultaneously measuring the pH, P.sub.CO2 and P.sub.O2 would be useful in other fluid analysis such as water pollution monitoring and general chemical analysis.
Furthermore, development of a microelectrode device would have a very wide application in clinical medicine. It could be used to perform in vivo determinations of the level of blood gases presnt in arterial blood. This would avoid the need to draw blood in order to make the measurement, a problem that becomes very significant in critically ill patients who cannot afford to lose a lot of blood due to frequent arterial blood gas tests. It would be desirable to make the microelectrode so simple that it could potentially be developed into a disposal arterial blood gas device. Instead of drawing blood from an artery into a disposable syringe, as it is presently done, the arterial blood gas levels could be determined directly in the artery or tissue without any blood loss. The microelectrode device could then be discarded if there is no need for continuous monitoring.
Therefore, the need exists for an electrode capable of simultaneously measuring the pH, P.sub.CO2, and P.sub.O2 of fluids, particularly a microelectrode capable of monitoring body fluids. It is desirable that such electrodes be easy to fabricate and also be produced from materials which would allow for the disposability of such electrode device after it has been used.